Field of the Invention
This invention relates to LED lighting units, and in particular to LED light fixtures having manually controllable correlated color temperature (CCT) to obtain different correlated color temperatures.
Description of the Prior Art
LED lamps incorporate light emitting diodes (LEDs) which are semiconductor light sources having a semiconductor diode and photo illuminescent material. An LED is a diode which permits the flow of electrical current in only one direction. In an LED, there are two conductive materials in contact with each other—a positive or p-material, and a negative or n-material. When a p-material is in contact with an n-material, a single p-n conductor is formed. The atoms in the n-type material have extra electrons while the p-type material has electron deficiencies or holes. When the p-type and n-type material are close to each other, the extra electrons of the n-type material transmit its extra electrons to holes in the p-type material. This results in the release of photons. However, only those diodes which release photons that are in the visible part of the light spectrum emit light and those diodes are LEDs.
LED lights have become very popular because they last longer and use far less electricity than incandescent bulbs. LED lights, particularly of the type disclosed herein, generate white light. White light is a combination of multiple wavelengths of light. One way of characterizing white light is by the use of color temperature. This means the color of light within the white light. CCT is a specification of color appearance of the light emitted by a lamp. It relates to the color from a reference source which is heated to a particular temperature, measured in degrees Kelvin (K). The CCT rating of a lamp in general refers to the warmth or coolness of the lamp. CCT is basically a gauge of how blue or yellow the color of light emitted from a source of illumination appears. Lamps with a CCT rating below 3200K are considered warm sources, whereas those above 4000K are considered cool sources. The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of comparable color to that of the light source. As mentioned above, color temperatures over 4000K are generally called cool colors (bluish white), while lower temperatures (2700K-3000K) are referred to as warm colors (yellowish white through red).
LEDS are usually either indicator-type LEDs if they have low intensity, and illuminator-type LEDs if they are high power devices that provide illumination—as is the case in the present invention. LEDS are classified in part according to their CCT. A color difference has a threshold at which it becomes observable based on an ellipse (called the MacAdam ellipse) over the color space so that the color at its center changes by a certain amount at any particular amount along its edge. The scale of this ellipse is referred to as the standard deviation of color matching or SDCM. A color difference of 2 SDCM is not readily visible. Color differences are intended to be small by light fixture manufacturers. LEDs are “binned” to classify the LEDs so that the CCTs of LED in a bin appear to be the same. The present invention uses LEDs of different CCTs; hence binning is significant.
Neutral white light is generally 4000K-4500K, direct sun is generally around 4800K, day-white is 5000K-6000K, cool white is generally 7000K-7500K and blue sky is considered to be 10000K. LED downlights have become more and more popular. However, while these lights can be mass produced and easily affordable, it has been desirable to incorporate easily operable, manual systems for controlling the CCT that is efficient, effective and inexpensive.
It has also been found very useful to incorporate such lights in recessed cans which are installed in many residences, commercial buildings, hospitals, educational institutions and the like.
For LED lamp fixtures, it has been found desirable to use LEDs with various CCTs because of different tones of light desired.
For example, there have been various recommendations for CCTs for different environments. With respect to restaurants, it is been recommended that high-end restaurants use warmer color temperatures from 1800K-2700K. Court-serve restaurants, warm, but not as warm as high-end restaurants, the recommendation is 2700K-3500K. For hotel lobbies, a recommendation in the 1800K-3000K range. For guest rooms in hotels, one recommendation is in the 2700K-3000K range to create a warm and inviting atmosphere. However, in offices, a recommendation is in the 3000K-4000K range to provide a neutral to cool color temperature. With respect to healthcare, such as in hospitals, cooler color temperatures have been recommended in the 3500K-5000K range. Of course, if a particular space is used for different purposes, it would be very advantageous to be able to change the CCT. Moreover, different individuals or groups of individuals may have their own personal preference for a particular range of CCTs. Thus, LED light fixtures with easily variable CCTs would be most advantageous.